The present invention relates to position detectors, and more particularly to digital position detectors, such as resolvers, which are capable of determining the position of an object by means of an electrical signal the phase of which is varied in proportion to the rotational angle of a detector shaft which is mechanically coupled to the object under measurement.
There are a great number of industrial applications in which precise measurements of the positions of objects are of paramount importance. Automatically controlled machines are typical examples, because the positions of the moving elements thereof must first be determined precisely in order that the moving parts of the machines be controlled and moved to the predetermined position. In the case of industrial robots or numerically controlled machine tools, for example, the rotational angles of the driving shafts which are mechanically coupled to the elements to be moved are measured to determine the positions of the elements.
Among the most common position detectors that are capable of determining the position of an object through the angle of rotation of the detector shaft coupled to the object the position of which is to be measured are those which utilize the phase modulation method. According to this method, the phase of an electrical signal is modulated in proportion to the angle of rotation of the detector shaft, and the determination of the rotational angle of the shaft is effected by measuring the phase angle of the electrical signal. A typical example of such position detectors utilizing the phase modulation method is the resolver.
The operational principle of a typical conventional resolver which measures the rotational angle of the detector shaft in the digital mode is as follows. The clock pulses generated by a clock are counted by a counter which counts, for example, up to 1000. The stator windings of the resolver are excited in precise synchronization with the content of the counter so that a magnetic field is generated which rotates in exact synchronization with the counter content and completes one rotation every time the counter counts to up to 1000. Thus, a sinusoidally varying electrical signal is generated in the rotor winding wound around a rotor. As the rotor is fixedly mounted on the detector shaft which is rotated, the phase angle of the electrical signal thus generated corresponds to the rotational angle to be measured.
The sinusoidal electrical signal generated in the rotor of the resolver is further processed to adapt the signal to digital mode operation by a wave shaper circuit to produce a square or rectangular pulse signal which takes the logical value 1 or 0 depending on whether the sinusoidal electrical signal is positive or otherwise. Thus, the length of time between the time point at which the counter takes the value 0 and the time point of the rising edge of the rectangular pulse signal corresponds to the phase angle of the sinusoidal electrical signal, and thus corresponds to the rotational angle of the detector shaft at the point of time at which the rising edge of the rectangular pulse signal occurs. Because this length of time, as measured by the aforementioned clock pulses, is equal to the number stored in the aforementioned counter at the time of the rising edge of the rectangular pulse signal, the content of the counter is transferred to a phase angle register every time the rising edge of the rectangular pulse signal occurs. Thus, the numerical value stored in the phase angle register represents the digital angle of rotation of the detector shaft measured digitally to one thousandth of a revolution.
The central processing unit reads out the numerical value stored in the phase angle register at fixed sampling periods, and calculates the overall angle by which the detector shaft has rotated from the beginning up to the present sampling time by means of the sampled values of the phase angles.
In the case of the resolver as described above, however, if the numerical value stored in the phase angle register changes by an amount greater than 500, i.e. one half revolution, during the time interval between the two successive sampling times, the central processing unit cannot determine whether the shaft has rotated forward or backward during the same interval. Thus, the determination of the overall angle of rotation of the detector shaft from the beginning up to the present, or, what amounts to the same thing, the determination of the position of the element coupled to the detector shaft, becomes impossible when the detector shaft rotates by an angle exceeding one half a revolution during the time interval between the two successive sampling times.
Further, the numerical value read out from the phase angle register at the sampling time corresponds to the rotational angle of the shaft at the time at which the preceding rising edge of the aforementioned rectangular pulse signal occurs, and thus does not represent the rotational angle which is taken by the shaft at the sampling time. The existence of this time lag between the sampling and the measurement time is also an obstacle to the precise determination of the position of the element coupled to the detector shaft.